1. Field of the Invention
The present invention relates to a process of preparing an improved solid catalyst for use in acid catalyzed organic reactions such as Friedel-Craft's reaction, nitration, cyclization and the method of manufacture of such a catalyst. This invention particularly relates to the preparation of catalysts for use in acid catalyzed organic reactions which occur in the microporous range of the catalysts such as nitration of aromatic compounds, cyclization of terpenoids and more particularly relates to the preparation of modified sulphated zirconia catalysts.
2. Description of the Related Art
Laszlo, P. and Pennetraeu, P., J. Org. Chem., 52, 2407, 1987 have shown that copper nitrate supported K-10 clay gives the best o:p ratio of 1:7.5 so far reported for nitration of chlorobenzene. However, this catalyst did not give adequate para product, which is an important drug intermediate.
Shabtai, J., Lazar, R. and Biron, E., J. Mol. Cat., 27, 35, 1984 have shown that depending on the alkali metal introduced in to the zeolite, the reaction yields either citronellol or isopulegol. With NaX, 85% isopulegol and 14% citronellol are obtained at 87% conversion of citronellal, while with CsX 92% citronellol is obtained at 77% conversion. However, this is rather attributed to a difference in pore dimension than to a difference in basicity.
Superacids as Catalysts
Superacids like K-10 clay, zeolites, silica-alumnia, sulphated metal oxides, like sulphated zirconia, etc. are substances known to have acidity higher than that of 100% sulphuric acid.
Preparation of Sulphated Zirconia (S-ZrO.sub.2)
A variety of methods have been reported for the preparation of sulphated zirconia. These methods differ mainly in the type of precursor, type of precipitating agent, type of sulphating agent, method of impregnation, calcination temperature, etc.
The type of precursor for preparing sulphated zirconia plays a vital role in the final texture and hence, the performance of the catalyst. Various zirconium compounds such as Zr(NO.sub.3).sub.4, ZrCl.sub.4, zirconium isopropoxide, zirconyl chloride, zirconium oxychloride and sometimes, zirconia itself are used to prepare these catalysts. Various precipitating agents like aqueous ammonium hydroxide, and urea have been reported (Yamaguchi, T. and Tanabe, K., Mater. Chem. Phys., 16, 67, 1986). Sometimes hydrogen sulphide and sulphur dioxide are also used as sulphating agents. Amorphous zirconium hydroxide obtained by the alkaline hydrolysis of the zirconia precursor is usually sulphated before it is crystallized by thermal treatment. The sulphating species most commonly used are sulphuric acid and ammonium sulphate (Sohn, J. R. and Kim, H. W., J. Mol. Catal. 52, 361, 1989). The sulphated species is then thermally crystallized whereby it undergoes phase transformation, the tetragonal phase being stabilized as a result of sulphate incorporation.
Sulphated zirconia as such, prepared by co-precipitation of zirconium oxychloride with ammonia followed by sulphation, is a highly superacidic catalyst. Sulphated zirconia, prepared by sol-gel method, is also highly superacidic. However, co-precipitated zirconia is cost effective. Many industrially important reactions have been studied for the use of sulphated zirconia because of its superacidic character. Some of these reactions are Friedel-Crafts alkylation, acylation, condensation, esterification, etherification, nitration, isomerization, cracking, dehydration, oligomerization, etc. But one of the major drawbacks of sulphated zirconia is that it is not a shape selective catalyst.
Carbon Molecular Sieves (CMS) and CMS Coated Catalysts
Carbon molecular sieves (CMS) are substances which have micro- or mesopores depending on the source and the method of preparation. They are mostly used in the separation of gases like nitrogen and oxygen gas from air, methane and ethane gas, etc. CMS are prepared from coal, by pyrolysis of precursor polymeric materials like polyacrylonitrile, phenol formaldehyde resin, polyvinylidene chloride, polyfurfuryl alcohol, polyvinyl alcohol, etc. or any combination of the above. CMS prepared by pyrolysis of polymers are found to be inert having absolutely no catalytic activity. The diffusivities of molecules through the ultramicroporous networks of CMS materials display a strong dependence on the critical kinetic diameter of the molecule. Hence the concept of coating carbon molecular sieves on superacids has been explored.
Foley, H. C., Perspectives in Molecular Sieve Science, American Chemical Society, 335, 1988, reports that the research groups of Walker and Trimm were the first to have investigated the reactant shape-selectivity of metal containing carbon molecular sieves. Trimm and Cooper (Chem. Commun., 477, 1970; J. Catal., 31, 287, 1973) and Schmitt and Walker (Carbon, 9, 791, 1971; Carbon, 10, 87, 1972), studied CMS/Pt catalysts for the shape selective hydrogenation of gas phase olefins. There were further developments in this field whereby a composite mixture of inorganic oxides (e.g., SiO.sub.2, TiO.sub.2, ZrO.sub.2, TiO.sub.2 --ZrO.sub.2, etc.) modified with CMS could be used as catalyst.